1. Field of the Invention
The present invention relates to a spark-assist type self-ignition engine.
2. Description of the Related Art
In the past, when injecting fuel in a diesel engine, part of the fuel immediately evaporates to form a preliminary air-fuel mixture. The remaining fuel is dispersed in the combustion chamber in the form of fuel droplets. Next, the preliminary air-fuel mixture is made to self-ignite. This serves as the heat source for successive combustion of the fuel evaporated from the fuel droplets. That is, diffusive combustion is performed. In this case, when the ratio of the preliminary air-fuel mixture becomes greater, explosive combustion occurs, so the combustion temperature rapidly rises and therefore a large amount of NOx is produced.
Therefore, known in the art is a diesel engine designed so that a combustion pressure sensor and spark plug are arranged in the combustion chamber, the ratio of the preliminary air-fuel mixture is found from the detected combustion pressure, when the ratio of the preliminary air-fuel mixture becomes greater than the optimum value, the spark timing is advanced to make the preliminary air-fuel mixture ignite by the spark plug before the amount of generation of the preliminary air-fuel mixture increases excessively, and when the ratio of the preliminary air-fuel mixture at the time of ignition by the spark plug becomes smaller than the optimum value, the spark timing is delayed to make the preliminary air-fuel mixture ignite by the spark plug when the amount of generation of the preliminary air-fuel mixture increases to a suitable amount (Japanese Unexamined Utility Model Publication (Kokai) No. 2-141648).
The light oil used in diesel engines has a large fuel component with a high molecular weight, so the majority of the fuel does not easily evaporate. When injected, only the part of the low boiling point component gradually evaporates. That is, after the start of the injection, the amount of the preliminary air-fuel mixture generated gradually increases along with the elapse of time. Therefore, in a diesel engine, as explained above, it has been considered to control the amount of preliminary air-fuel mixture able to be ignited by control of the spark timing.
Gasoline, methanol, and other low cetane value fuels inherently do not easily self-ignite, but even if these fuels are used, they will self-ignite if the gas temperature in the combustion chamber becomes high enough. This self-ignition, however, cannot be controlled unlike with a diesel engine. That is, gasoline and methanol have low molecular weights, therefore when injected immediately vaporize whereby the entire air-fuel mixture becomes a preliminary air-fuel mixture. Accordingly, when self-ignition occurs, the entire air-fuel mixture always self-ignites. That is, it is not possible to control the ratio of the preliminary air-fuel mixture causing self-ignition as in a diesel engine.
The self-ignition when using gasoline or methanol, however, occurs before the top dead center of the compression stroke, therefore when self-ignition occurs, knocking occurs. Therefore, in the related art, when using a fuel such as gasoline or methanol, effort is made to prevent self-ignition as much as possible and to make the air-fuel mixture burn by the flame of ignition of the spark plug.
Since the entire air-fuel mixture starts to burn simultaneously when self-ignition occurs, the temperature does not become higher locally. The combustion temperature in the combustion chamber becomes lower overall. As a result, the amount of generation of NOx becomes extremely small. Accordingly, if it were possible to control the timing of the self-ignition so that the self-ignition occurred after the top dead center of the compression stroke, gentle combustion with little generation of NOx could be achieved.
The timing of the self-ignition is controlled even in the above diesel engine, but in a diesel engine the timing of the self-ignition is controlled to control the preliminary air-fuel mixture. As opposed to this, in the present invention, the timing at which the entire air-fuel mixture self-ignites is controlled, so the above-mentioned diesel engine and the present invention inherently differ from each other.